This invention relates to plasma reactors and, in particular, to coupling radio frequency (RF) energy into the reactor volume.
In the prior art, plasma reactors have been used for dry chemical treating of a variety of products, including semiconductor wafers. In all of these processes, whether for etching or depositing, RF power must be coupled into the reactor to create a plasma discharge. The plasma discharge creates the reactive species which are used to treat the wafer. The RF power can be coupled into the reactor either inductively or capacitively. Virtually all current reactors use capacitive coupling for RF energy.
As with any other system handling RF energy, the efficiency of the coupling depends upon the impedance match of the components. In a typical plasma reactor, RF energy is coupled to the lower electrode. The lower electrode is moveable to facilitate loading and unloading wafers. Thus, the RF power is necessarily coupled to the electrode by way of a coaxial cable which is much longer than the actual distance between the RF amplifier and the reactor chamber and is subject to frequent flexing as the reactor is operated. Further, as the result of operating the reactor, even with etch processes, deposits form on the electrode which precludes a simple mechanical contact as a reliable means of exciting the moveable electrode. Because of the length of the coaxial cable and the flexing, it is quite likely to leak RF, which must then be contained by the enclosure of the reactor. These problems are made more difficult by the fact that a plasma reactor looks like a high impedance which results in a high VSWR in the flexible cable with resulting power loss, heating of the cable, and spurious radiation.
In view of the foregoing, it is therefore an object of the present invention to provide improved RF coupling to a chamber.
Another object of the present invention is to provide a mechanism for capacitively coupling RF energy into a plasma reactor.
A further object of the present invention is to capacitively couple RF energy to a moveable electrode.
Another object of the present invention is to eliminate coaxial cable connections to a moveable electrode.
A further object of the present invention is to automatically disable a plasma reactor upon opening the chamber.
The foregoing objects are achieved in the present invention wherein a plasma reactor chamber contains a coupling ring electrically insulated from the conductive chamber walls. A conductive lower electrode, when moved into position to close the chamber, is adjacent the ring and separated therefrom by an insulator. The electrode and ring have an annular area of overlap and thus form two plates of a capacitor and form a coupling capacitor for applying RF energy to the chamber. When the chamber is opened the capacitor no longer exists and no RF power is coupled to the lower electrode.